There are a number of applications requiring accurate frequency and/or time synchronisation references in order to properly operate, for example mobile technologies such as Global System for Mobile Communication (GSM), Wideband Code Division Multiple Access (W-CDMA) and Long Term Evolution (LTE). In case of frequency synchronisation the traditional solution is to get synchronisation from a synchronous stream of data, as for instance in case of Time-Division Multiplexed (TDM) based networks, but the migration of networks from TDM to packet-based technologies (such as Ethernet) requires a different approach.
One solution is to use a packet based method, where the timing is carried across a packet network by sending packets containing timestamp information. The timestamps are generated by a master (server) that has access to an accurate reference, such as Global Positioning System (GPS).
It is possible to provide time synchronisation between a pair of nodes by using a timing protocol such as the Network Time Protocol (NTP) or Precision Time Protocol (PTP), defined in IEEE 1588. A master node with access to an accurate time source (e.g. GPS) provides a timestamp and the protocol determines the transfer delay between the master node and the slave node. One fundamental assumption with this approach is that the delay from master to slave and from slave to master shall be identical. This means that any asymmetry in the network would significantly impact the performance of the delivered time synchronisation reference.
If the delay asymmetry of the path connected to the ingress port is known, the corrections can be made as specified by the PTP protocol. In particular IEEE 1588 defines an attribute “delayAsymmetry”.
In order to handle the packet delay variation and the asymmetries in the network the “Boundary Clock” or “Transparent Clock” functions have been specified by IEEE 1588. The IEEE 1588 transparent clock is a function that provides a means of measuring the delay that has been added by the network element and of measuring the delays on links connected to the network element. The end-equipment can use this information to recover the time reference. The boundary clock, by contrast, terminates and regenerates timestamp packets. While any asymmetry in the node is effectively removed by means of the HW timestamping at the ingress and egress ports, still asymmetries may be present in the links connecting two nodes.
This may happen in case of forward and reverse traffic (and therefore PTP flow) in the same fibre but over different wavelength (e.g. WDM-PON) or in case of forward and reverse traffic in two different fibres (and using the same wavelength), where the fibres may have different transmission characteristics and different lengths.
A known solution to correct for asymmetries in the links is to manually calibrate the links. If the delay asymmetry of the path connected to the ingress port is known, the corrections can be made as specified by the PTP protocol. However, this must be performed node-by-node and can be an extremely costly and time consuming process. Moreover, at any change in the network (e.g. adding transmission equipments) the compensation has to be updated. This can be a too complex and costly task creating a significant obstacle in the deployment of IEEE 1588 technology.